Systems have been suggested heretofore for obtaining computed tomography (CT) scans for medical or other purposes. In general, these known systems are comparatively complex structurally, very expensive, and tend to subject a patient to a comparatively high dosage level of radiation if X-ray images of adequate quality to effect an X-ray diagnosis are to be obtained. The present invention is concerned with the provision of an apparatus which, when employed as a CT scanner, is capable of producing X-ray images which are comparable to and in some cases better than those produced by present-day commercial CT equipment, and which achieve these results at far less cost and by subjecting the patient to a far smaller level of dosage than is customary at the present time. These advantages are achieved by the provision of equipment which employs a mechanical scanner, of the general type described in Stein et al U.S. Pat. No. Re. 28,544 (originally U.S. Pat. No. 3,780,291) which is operative to produce a pencil beam of X-rays that scans a single efficient detector.
CT equipments employing flying spot scanning techniques have been suggested heretofore. One such arrangement is described, for example, in an article entitled "Low-Dosage X-Ray Imaging System Employing Flying Spot X-Ray Microbeam (Dynamic Scanner)" by Tateno and Tanaka, Radiology 121: October 1976, pp 189-195. The Tateno et al system, although described as being capable of achieving quality X-ray images at lower dosages than are customarily employed in CT equipment, uses a special noncommercial X-ray tube characterized by sophisticated electron optics analogous to those employed in high voltage electron microscopes and electron beam machining equipment, relies on an electronic scanning technique, and contemplates the use of a two-dimensional detector. These characteristics of this previously-described system make the system far more expensive than the system of the present invention, which utilizes an extremely simple mechanical scanning arrangement. In addition, inasmuch as the Tateno et al system employs a two-dimensional detector, it is incapable of rejecting scattered radiation, in contrast to the system of the present invention wherein, by use of a single, efficient one-dimensional detector, such rejection is automatically accomplished.
Further advantages accrue to the present invention, as compared to the scanning techique of Tateno et al which employs a device that produces a flying-spot X-ray beam by "pinhole" projection of an electronically scanned focal spot in the X-ray tube. In order to produce an X-ray field large enough to subtend a patient cross-section for a CT scan, the beam must diverge over a considerable distance from the pinhole collimator. The required distance is equivalent to locating the pinhole at the focal spot (X-ray source) of the present invention. Since the beam cross-section at any point represents a pinhole image of the focal spot, the relatively large distance from pinhole collimator to patient results in a relatively large beam cross-section, with a concurrent loss of resolution. The close proximity of the collimation systems to the patient in the present invention is an important improvement, since the beam size is essentially a projection of the small collimator apertures from a distant source.
Another system suggested heretofore, for producing CT images by use of a flying spot technique, is described in Hounsfield U.S. Pat. No. 3,866,047 for "Penetrating Radiation Examining Apparatus Having A Scanning Collimator". The Hounsfield apparatus contemplates the provision of a mechanical scanning device comprising a pair of elongated shutters which are mounted for mechanical reciprocation in synchronism with one another. Each shutter member is provided with a plurality of slots which coact with one another to produce a plurality of angularly spaced radiation beams simultaneously, each beam being caused to scan through a comparatively small angle onto a comparatively small detector which is associated with that beam. The Hounsfield reciprocating shutter arrangement is far more complex mechanically than the comparatively simple rotating collimator which is employed in the present invention, and requires critical alignments of the plural slots which are utilized in the spaced shutters of the Hounsfield mechanical scanner. Moreover, since Hounsfield contemplates the simultaneous generation of a plurality of angularly displaced X-ray beams, and the simultaneous scanning of all of those beams across a like plurality of detectors, the arrangement poses problems of possible loss of data at the boundaries between adjacent detectors. Two specific problems may be identified: (1) The boundaries produce a geometric inefficiency which results in wasted dose to the patient, and (2) The missing information along the beam paths through the boundaries can result in artifacts in the reconstructed CT image.
Further problems with the multiple beam arrangement of Hounsfield are related to the need for accurate matching or normalization of the plural detectors over the full dynamic range of the signal, without which severe artifacts can result in the reconstructed image. A number of phenomena, as for example cathode resistivity and dynode fatigue, are known to produce nonlinearities and gain changes in photomultiplier tubes, the use of which is contemplated by Hounsfield. Similar problems may occur with other plural detectors which are less efficient than the scintillator-photomultiplier combination. In order to reduce the dynamic range, and thereby alleviate the normalization, Hounsfield has incorporated a "plastics block" (item 26 in his Figures) and suggests the use of a water bag filling the space between the plastics block and the patient. The use of such devices introduces extra expense and mechanical complexity, and results in wasted dose because of photon absorption (and consequent loss of information) between the patient and the detector.
The present invention utilizes a single, efficient detector and a simple mechanical scanning arrangement to obviate all these problems of the prior art.